In many wireless communications systems, and especially in cellular communication systems, it is important to control the transmitted power of a traffic channel in order to reduce cochannel interference. Cochannel interference is generated by other transmitters assigned to the same frequency band as the desired signal. And because all users transmit traffic on the same carrier frequency in a code division multiple access (CDMA) cellular system, reducing cochannel interference in CDMA systems is especially important because it directly impacts system capacity. If the cochannel interference is reduced, the CDMA system capacity may be increased. Therefore, it is a design goal to transmit a traffic signal with only an amount of power necessary to provide acceptable signal quality at the receiver, after it passes through the channel.
In this document, a "channel" may be defined as a path or paths of communication through a medium between a transmitter and a receiver. If the medium is air and communication takes place with radio frequency (RF) signals, such a channel is typically affected by fading. A "traffic channel" may be defined as a channel that carries data, whether representing voice or other information generated by the user, which the user intends to transmit via the channel. The traffic channel may be distinguished from other channels used by the communication system, such as channels that may be used to transmit timing, control, or other information supporting system operation.
Power control systems in cellular communication systems should compensate not only for signal strength variations due to the varying distance between the base station transceiver and the subscriber unit, but should also attempt to compensate for channel quality fluctuations typical of a wireless channel. These fluctuations are due to the changing propagation environment between the transmitter, or base station, and the receiver, or subscriber unit, as the user moves in the service area.
Existing power control systems used in CDMA cellular systems that operate according to J-STD-008, published by the Joint Technical Committee on Wireless Access, use the measurement and reporting of cyclic redundancy check (CRC) errors at the subscriber unit to control the power of the traffic channel at the base unit. This method of power control in response to CRC errors is used to implement a slow "ramping" power control scheme. The "ramping" occurs because the traffic channel power is increased by a relatively large amount when the subscriber unit reports CRC errors. After the large power increase, which often eliminates the CRC errors for some subsequent period, the power is reduced by a relatively small amount for each subsequent frame transmitted. Eventually, the power is reduced to a point where another CRC error occurs, and the power is once again increased by a relatively large amount. If channel quality remains constant, a graph of power transmitted in the traffic channel resembles a saw tooth, with large power increases followed by a series of small power decreases.
One problem with this method of power control is the delay encountered between the degradation of channel quality and the request for a power increase and the subsequent actual increase in power. The delay in requesting a power increase is caused by waiting for a frame to be received, and then waiting for frame decoding and the detection of a cyclic redundancy check error. Once the CRC error is detected, it must be reported to the base station, and the base station must respond by increasing traffic channel power. In current CDMA systems, it takes 20 milliseconds (mS) to receive a frame. Thus, the rate at which CRC reports or power control commands are sent to the transmitter is 50 Hz. This delay in the power control loop periodically causes the base to transmit too much power on the traffic channel, such as when a relatively large increase in power is requested and granted just as the channel quality has reached a minimum and starts to improve. If the traffic channel has too much power, cochannel interference increases and system capacity decreases.
With reference now to FIG. 1, there are depicted relevant portions of transceiver 20 that uses orthogonal transmit diversity (OTD). As illustrated, traffic channel data source 22 provides a stream of symbols, which may represent voice or data traffic of a plurality of users or channels. The rate that symbols are output from data source 22 is controlled by symbol clock 24.
Symbols from traffic channel data source 22 are convolutionally encoded by convolutional encoder 26. Convolutional encoder 26 encodes at a rate of one divided by "n". This means that for every symbol entering convolutional encoder 26, n encoded symbols are output. Clock multiplier 28 provides a clock for convolutional encoder 26 that is n times the rate of symbol clock 24.
After traffic channel data symbols have been encoded, power control encoder 30 places uplink power control information into the stream of encoded symbols. In one proposed system, this is accomplished by inserting a power control bits in a predetermined bit locations in power control groups of a frame in the data stream. Thus, some traffic channel data bits are replaced, or punctured, by bits intended to direct the subscriber unit to raise or lower its transmit power level. The frequency at which power control bits are punctured remains at a predetermined frequency, which in a preferred system is 800 Hz. Additionally, the power level of the punctured power control bits are set at the full vocoder rate traffic power level. The power control bits are preferably evenly distributed among the transmit antennas.
After the power control bits are inserted in the data stream, commutator 32 distributes symbols among diversity branches of the orthogonal transmit diversity transmitter. As shown in transceiver 20, there are two diversity branches defined by paths through spreaders 34 and 36, which paths use different spreading codes to spread the symbols in each branch.
After the symbols are spread with multiple orthogonal spreading codes, the spread data outputs are amplified by amplifiers 38 and 40. Amplifiers 38 and 40 are coupled to power controller 42 which controls the gain of amplifiers 38 and 40.
The outputs of amplifiers 38 and 40 are each coupled to separate antennas 44 and 46, which provide different signals that propagate through different paths r.sub.1 and r.sub.2, before they may be received by a subscriber unit. Also note that one or both antennas, such as antenna 46, may be used to receive power control information PC transmitted from a subscriber unit. This power control information is coupled to power controller 42 so that power controller 42 may set the gain of amplifiers 38 and 40.
With reference now to FIG. 2 there is depicted selected portions of a subscriber unit 50 according to the prior art. As shown, antenna 52 receives signals through paths r.sub.1 and r.sub.2, which carry traffic channel data and other control data. Antenna 52 is also used to transmit power control information PC to transceiver 20 shown in FIG. 1.
Antenna 52 is coupled to down converter and demodulator 54, which down converts and demodulates the received signals.
The output of downconverter and demodulator 54 is split to form diversity branches within subscriber unit 50. These diversity branches correspond to the antennas and diversity branches within transceiver 20. Thus, transceiver 20 in FIG. 1 is shown with two diversity branches, and subscriber unit 50 is also shown with two corresponding diversity branches.
The paths along diversity branches pass through despreaders 56 and 58, respectively. These despreaders use despreading codes similar to the spreading codes used in transceiver 20.
Clock recovery circuit 60 is also coupled to the output of downconverter and demodulator 54. Clock recovery circuit 60 produces a symbol clock that is used by decommutator 62 to reassemble the symbol stream within subscriber unit 50. The symbol clock is also used by power control group clock 63 to generate a clock having a frequency set at a power control group rate. Since in a preferred embodiment there are 16 power control groups in a 20 mS frame, the power control group rate may be 800 Hz.
The output of decommutator 62 is input into a decoder, such as Viterbi decoder 64, for decoding the convolutionally encoded data. Following decoder 64 CRC circuit 66 performs a cyclic redundancy check on a frame of data to determine whether or not an error has occurred. The output of CRC circuit 66 is coupled to outer-loop threshold circuit 68, which adjusts an outer-loop, or slower loop, threshold, which helps subscriber unit 50 maintain a selected frame error rate.
Subscriber unit 50 also uses fast power control, which is controlled by a faster inter-loop feedback mechanism which comprises signal-to-noise measurers 70 and 72 and an arithmetic mean calculator 74. In a preferred embodiment, signal-to-noise measurers 70 and 72 are coupled to the diversity branches of subscriber unit 50 for measuring a channel quality, such as a diversity signal-to-noise ratio, for each diversity branch. The outputs of signal-to-noise ratio measurers 70 and 72 are coupled to a arithmetic mean calculator 74, which calculates the arithmetic mean of the measured signal-to-noise ratios by adding them together and dividing by the number of signal-to-noise ratios. The arithmetic mean output by arithmetic mean calculator 74 is coupled to comparator 76, which outputs a power control bit to instruct transceiver 20 to increase or decrease transmit power. This information is transmitted from antenna 52 as shown at signal PC, which is also received at antenna 46 in transceiver 20 (see FIG. 1).
Note that arithmetic mean calculator 74 operates at an inter-loop rate, which in a preferred embodiment is 800 Hz, while CRC circuit 66 and outer loop threshold circuit 68 operate at a frame rate, which in a preferred embodiment is 50 Hz. Clock divider 78 is used to divide the clock down and set the relative clock rates between the inter-loop and outer-loop.
The fast power control system shown in the orthogonal transmit diversity system that includes transceiver 20 and subscriber unit 50 malfunctions when one of radio frequency paths r.sub.1 and r.sub.2 is in a deep fade, and when the rate of convolutional encoder 26 is rate one-half. In this case, half the symbols, which are transmitted from one antenna, are lost due to the fade, and the symbols received from the other antenna are received with CRC errors, or frame errors, due to power control bits punctured by power control encoder 30. Thus, every frame is received with an error in subscriber unit 50, which causes outer loop threshold 68 to be adjusted rapidly so that it is soon requesting maximum transmit power from amplifiers 38 and 40 in transceiver 20. When this occurs, the system assumes a malfunction and the call is dropped. Therefore, it should be apparent that a need exist for an improved method and system for generating a power control metric in an orthogonal transmit diversity communications system.